I have an old (20+ years old) Nikon mount Sigma 50mm f/2.8 macro lens with a 2X Tamron teleconverter that I use on my Canon body. When taking macro (1:1) or general close-up shots, the lens is very sharp and quite usable even with the TC.

However, when I take photos of further away subjects, especially when attempting to use the lens while experimenting with landscapes, the lens is soft and not very nice at all. In fact, even stopping down to f/8 or f/11 doesn't improve things much.

I know that the current breed of Canon and Nikon macros are incredibly sharp, even at far away focus distances, making them suitable for portrait use. Is this a recent phenomenon? Did old macro lenses exhibit softness when focused at further away objects?

How are you getting from Nikon to Canon mount? Between the teleconverter and the body , between the lens and the teleconverter? Or is the Tamron made with different mounts on each end?
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Michael ClarkApr 6 '13 at 6:35

Adapter between the teleconverter and the body.
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Chinmay KanchiApr 7 '13 at 9:36

Have you tried using the lens straight to the adapter without the teleconverter?
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Michael ClarkApr 7 '13 at 11:13

Yeah, that helps but not as much as I'd expect. Macro sharpness improves a great deal; long distance, not so much.
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Chinmay KanchiApr 7 '13 at 13:12

2 Answers
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That would depend on the individual lens, but to make a general statement: no, direct-mount macro lenses that could focus to infinity were generally sharp throughout their focal range. In particular, a macro lens in the 90-105mm range was usually the sharpest lens in a photographer's kit, regardless of the brand (within limits, of course—there have always been $25 specials of dubious origin, whether on eBay, Amazon, or in the black-and-white ads at the backs of photo magazines).

I qualified that somewhat, since there were lenses produced that either required a focusing bellows or that could only focus at all at relatively close distances. Some of them were adaptations of microscope optics; others were highly corrected for apochromaticity (a near-complete absence of chromatic aberration of any kind) and rectilinearity over a relatively small reproduction range (around 1:4 to 4:1), and while that made them spectacular within their range, it would mean they were less than spectacular outside of it. Such lenses generally couldn't be focused farther away than their far-focus limit without deliberate action on the part of the photographer (using short extension tubes rather than bellows, "freelensing", etc.).

That's not to say that no designs were ever compromised for cost and simplicity. In order to maintain correction across a larger-than-normal focus range, some elements of a lens have to be able to move in relation to other elements, even in a unit focus design. (That is, on a lens where focus is achieved by moving the majority of the lens elements, as a group, further away from the film plane/sensor. Unit focus lenses will get physically longer as you focus closer.) And the corrective elements often need to be aspherical (exotic shapes) or have low- or anomalous-dispersion characteristics (exotic materials, like fluorite or expensive-to-produce and hard-to-work-with optical glasses). If the lens was particularly inexpensive, something had to give somewhere, and if the lens was primarily aimed at macro at a time when owning a 50mm "kit lens" was a given, then it is possible that any compromises made were towards the infinity-focus end of the lens's range.

It almost sounds like the adapter is too long and is adding too much spacing between the flange and the lens, which results in the same effect as adding a short extension tube: minimum focus distance is shortened, but at the cost of not being able to focus to infinity.

It could also be that the lens was designed to focus at a shorter MFD at the expense of distant focusing. Some, but certainly not all, older macro lenses were designed to be macro "only" lenses.

Infinity focus works fine, though the lens will also focus slightly past infinity. The images aren't out of focus, just soft. For the same reasons, the lens isn't "macro only", though it is clearly optimised for that purpose.
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Chinmay KanchiApr 7 '13 at 14:43